A hard disk drive generally includes a stack of rotating disks or platters, a spindle motor which causes the disks to rotate, read/write heads which fly above the surface of the disks, an actuator motor (known as a "voice coil motor" or VCM) which controls the positioning of the read/write heads, power circuitry to provide electrical power to the spindle and voice coil motors and control circuitry to control the operation of the spindle and voice coil motors.
A read/write head reads data from the disks by sensing flux changes on the magnetic surface of the disk as it passes beneath the read/write head. To synchronize the data being read from the disk with the operation of the data processing circuitry, it is required to carefully control the rotation of the disks and provide a smooth and silent operation. This is usually accomplished by controlling the current delivered to the spindle motor.
Current control can be effected in two ways. The first is pulse width modulation, in which a driving voltage is modulated by a square wave. The duty cycle of the pulse width modulation signal determines the average voltage applied to the spindle motor. In turn, the voltage determines the amount of current delivered to the spindle motor. The other mode of current control is known as linear current control. In this mode, an analog voltage input signal is provided which is proportional to the current to be delivered to the spindle motor. The spindle motor control circuitry processes the input signal and adjusts the level of current delivered by the power circuitry accordingly.
The three-phase brushless motor is one of the most widely used types of spindle motor which has current energizing respective coils using a full wave bridge configuration. The bridge includes two power stages, one for each phase, so typically there are six power stages each with a power device. Three of the power stages and their power devices are referred to as "low side" stages and devices because they are connected between the motor coil and ground. The other three of these power stages and their power devices are referred to as "high side" stages and devices because they are connected between the power supply and the motor coil.
The power devices are operated as switches in a sequence that allows pulses of current to flow from the power supply through a high-side power device, a coil of the first of the three stages, a coil of the second of the three stages, and then through a low-side power device to ground. The power device may include a power driver or FET. This process is repeated in a generally well-known manner for the other power devices and coil pairs to achieve three-phase energization from a single, direct current, power supply. The switching or commutation characteristics of the power devices are very important in achieving good performance from the motor and other favorable characteristics.
Conventional control circuits produce, at the driver outputs, rectangular driving pulses, causing in the windings current pulses having a current path that depends on the amplitude of the driving pulses, the winding inductance, and on the motor voltage (emf voltage). The rapid voltage and current changes at the driver outputs, caused by the rectangular driving pulses, lead to kick-back pulses or "flyback pulses" which are caused by a sudden magnetic discharge of an abruptly turned-off inductor, as it is generally known.
Rapid current changes and the flyback pulses cause problems. On one hand, they cause loud motor noises that are disturbing for the user, for example, of any hard disk drive. On the other hand, due to their high-frequency content, the flyback pulses cause strong electromagnetic radiation. This may lead to considerable disturbances in the apparatus equipped with the motor as well as in other apparatus. The heads may be located near to the disturbances. In order to counteract such disturbances, a known control circuit for a brushless motor has an RLC filter connected between each driver output and the associated winding terminal of the motor. By means of these filters, it is possible to slope the edges of the driving pulses reaching the motor windings. In this manner, it is possible to avoid the occurrence of electromagnetic disturbances on the lead wires to the motor windings as well as on the motor windings. However, due to the inductances of the RLC filters, flyback pulses are still present at the driver outputs of the control circuit. Thus, disturbing electromagnetic radiation still occurs at the site of the driver outputs.
Additionally, efficient motor drive requires that the excitation current in the three motor phases be aligned with the BEMF generated by the three phases. One scheme for achieving this alignment is the use of a phase-locked loop (PLL). The PLL adjusts the phase and frequency of the commutation so that the BEMF of the undriven (tri-stated) windings passes through zero in the center of the appropriate commutation state. This scheme works well when the shape of the commutation waveform includes an undriven region, as in a conventional 6-state, +1, +1, 0, -1, -1, 0, sequence. In addition to the undesirable acoustic noise, this step-function tri-stating of the undriven motor phases, together with the step-function driving waveform produces a degree of torque ripple in the motor. This torque ripple results in an unevenness or jerkiness in the motor rotation, which also excites resonances in the motor, causing undesirable acoustic noise.
Thus, if it is desired to reduce acoustic noise, a sine wave shaped excitation signal may be more appropriate than a 6-state sequence. If the motor driver consists of a sinusoidal current source, the same voltage sensing PLL described above can be used. In the case of PWM sinusoid drive, the motor phases are repetitively driven between ground and the supply. As a result, no bemf information can be extracted from them as input to the PLL. Nevertheless, PWM drive is highly desirable in order to minimize power dissipation in the driver IC. This permits lower cost packaging and an overall saving in system cost.
There has been recent emphasis on disk drive manufacturers to reduce the noise associated with disk drive motors. Consequently, what is needed is a disk drive and method for operating it in which the noise associated with the drive in operation is reduced or eliminated.